1. Field of the Invention
This invention relates to computer graphics systems and, more particularly, to methods and apparatus for storing signals appearing in both interlaced video and non-interlaced graphics modes in a single frame buffer.
2. History of the Prior Art
It is the vision of many that in the near future a person sitting at a personal computer will be able to call information from a number of different sources. For example, it is expected that a person will be able to hear telephone and radio communications, view television or recorded motion pictures, play stereo recordings of music, and operate computer graphical and text programs. It is also expected that all of these operations will be possible at the same time so that, for example, a television program may appear in one window of a computer display while a computer graphics program is running in another window, or computer graphics material may appear as an overlay on the television program.
It is much easier to visualize the results that one would like to reach than to reach those results, especially where the results require the combining of television (video) signals with computer graphics signals on the same output monitor. The crux of the problem is that, although both types of signals are electrical, they arrive in entirely different formats for their two purposes. The computer signals are digital while the television signals are analog and must first be converted to digital representations for presentation on a computer monitor. Moreover, the television signals (video) are presented at a different frequency than are the computer signals. The television signals appear in an interlaced pattern consisting of a first field having a reduced number of lines followed by a second field having the same reduced number of lines approximately one-sixtieth of a second later which is combined with the first to form a complete picture. The reason for the interlaced display is that it allows a less expensive monitor to present pictures which are entirely acceptable for television purposes. However, such a monitor is not acceptable for computer graphics where much more detail must be displayed and manipulated. Today, a typical computer display presents at least twice as many lines of data in a non-interlaced mode.
Thus, the data from these two different sources of two different types, interlaced and non-interlaced, must somehow be presented in a form which can be handled by a personal computer. The usual method suggested in the prior art is to convert the analog video data to digital data and place it in a first frame buffer, place the computer graphics data in a second frame buffer, and somehow switch between the two frame buffers in presenting the data to an output monitor. However, the video data stored in the video frame buffer is still in the interlaced form in which it came from a television or similar source while the computer data is stored in non-interlaced form in its frame buffer. The visionary also expects to be able to present both the video and the computer graphics held in these frame buffers together on either an interlaced television type monitor or a computer monitor of some sort. Thus, interlaced video data and non-interlaced computer data must somehow be intermingled and displayed on both interlaced and non-interlaced monitors at the option of the operator.
In order to deal with such different forms of data, it is necessary for the computer to recognize that the data is in one or the other of the two forms. The dual frame buffers provide this designation for the different types of data. Although the arrangement does provide, through its use of separate frame buffers, for easy determination of which information is video and which is graphics, it still requires circuitry or software for assigning the different types of information to different parts of the display. Moreover, in order to handle the transfers of information from interlaced to non-interlaced and vice-versa and to determine the information which is to be displayed at boundaries between the different types of information, it is necessary for a system to know the type of information in any position surrounding any pixel to be displayed. Such information allows the system to know what to do with the pixel information. Then the pixel information must be differently handled depending on the type of information, the boundary condition involved, and the type of display on which it is to be presented.
Ultimately, the prior art arrangements emphasize the single largest cost in designing computer devices, the cost of the memory associated with such devices. By using a full frame buffer to store each of the video and computer graphics inputs, the cost of the frame buffer memory is doubled over the cost for a conventional computer arrangement. Added to this is probably the cost of an additional buffer memory to store information regarding the type of data assigned to an area of the display.